Silicon, as an element, is widely used as a semiconductor material. Semiconductor grade silicon is of extremely high purity and even the most minor impurities can have very dramatic effects upon the semiconductive characteristics of the material and upon potential uses of such silicon. The electrical properties of silicon semiconductor devices are in large part determined by minute amounts of preselected impurities introduced in predetermined amounts into prelocated portions of a semiconductor device.
Epitaxial silicon offers the device engineer homogeneously doped single crystal layers with steep concentration gradients between the layers. Epitaxial films are grown in a variety of thicknesses
______________________________________ Thin 0.5-3 microns Medium 3-10 microns Thick 10-20 microns Very thick &gt;20 microns and doping levels Very heavily doped 0.005-0.1 ohm-cm Heavily doped 0.1-0.3 ohm-cm Moderately doped 0.3-1 ohm-cm Lightly doped 1-10 ohm-cm ______________________________________
Process difficulty increases considerably at the extreme values of both thickness and resistivity; n-type epitaxy production exceeds p-type by a wide margin.
Device applications include power, small signal discrete, bipolar memory, linear, microwave, imaging, and certain MOS devices. ("Semiconductor Silicon" 1973, ECS Softbound Series, New York 1973.)
The critical parameters for epitaxial film include thickness and resistivity, auto-doping and out-diffusion, crystallographic defects, buried layer pattern shift/washout, morphological defects. These parameters are controlled by:
______________________________________ Reactor Geometry Buried Layer preparation Reactor Pressure technique Method of Heating Growth Rate Substrate Orientation Growth Temperature Substrate Crystal Main Gas Purity perfection Growth Chemicals ______________________________________
Both silicon homo-epitaxy and silicon hetereo-epitaxy are recognized as well as other epitaxial films.
At atmospheric pressure, epitaxial silicon is grown in a hydrogen atmosphere from four chemicals as noted in Table I.
TABLE I ______________________________________ Epitaxial Growth of Silicon in Hydrogen Atmosphere Chemical Normal Temperature Allowed Deposition Growth Rate Range Oxide Level ______________________________________ SiCl.sub.4 0.4-1.5 u/min 1150-1250.degree. C. 5-10 ppm SiHCl.sub.3 0.4-2.0 1100-1200.degree. C. 5-10 ppm SiH.sub.2 Cl.sub.2 0.4-3.0 1050-1150.degree. C. &lt;5 ppm SiH.sub.4 0.2-0.3 950-1050.degree. C. &lt;2 ppm ______________________________________
The thermodynamics of the H.sub.2 -Si-Cl system are well characterized. Other silicon halide systems have been studied (E. A. Taft, J. Electrochem. Soc., vol. 118, p. 1535, 1971; L. P. Hunt and E. Sirtl, Abs. No. 270, pp. 672-673, Extended Abstracts, 72-2, Electrochem. Soc., New York, 1972; T. Suzuki, M. Ura, T. Ogawa, pp. 191-200, Proc. Conf. Chem. Vap. Dep., Second Conf., J. Block & J. C. Withers, Eds., ECS Softboud Series, New York 1972); however, chlorine chemistry is most widely used. HCl impurities have been reported to affect device properties and there are reports of significant reduction of impurities when bromine-based chemicals are used (L. V. Gregor, T. Balik, F. J. Campagna, IBM J. Res. and Dev., vol. 9, p. 327, 1965); however, these benefits have not been realized for most devices in production today.
Epitaxy can be defined as the regularly oriented growth of one crystalline substance on another. The growing crystalline substance is called an "epitaxial film", while the substance upon which the growth occurs is known as the "substrate". If the epitaxial film and the substrate are substances of the same kind, one deals with homoepitaxy, if not, heteroepitaxy. In addition, an epitaxial film can be grown with material supplied from a liquid phase or vapor phase in contact with the substrate. Thus, growth across a solid-liquid interface occurs in liquid-phase epitaxy, while a solid-vapor interface is employed in vapor phase epitaxial growth.
Currently employed commercial processes for vapor phase growth of silicon films are shown in Table II.
TABLE II ______________________________________ Commercial Processes Source Materials Deposition Temperature Film ______________________________________ SiCl.sub.4, H.sub.2 1200.degree. C. Epi SiHCl.sub.3, H.sub.2 1150.degree. C. Epi SiCl.sub.2 H.sub.2, H.sub.2 1100.degree. C. Epi SiH.sub.4, H.sub.2 1050.degree. C. Epi SiH.sub.4, He 900.degree. C. Epi SiHCl.sub.3, H.sub.2 900-1000.degree. C. Poly SiH.sub.4 600-700.degree. C. Poly (low pressure) ______________________________________